1. Field of the Invention
The present invention relates generally to a polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte containing a non-aqueous electrolyte solution, and more particularly, to a polymer electrolyte battery using such a polymer electrolyte characterized in that preservation characteristics of the battery are improved upon improvement of the non-aqueous electrolyte solution contained in the polymer electrolyte.
2. Description of the Related Art
Recently, as one type of advanced batteries featuring high power and high energy density, non-aqueous electrolyte batteries with high energy density have been used. The non-aqueous electrolyte battery employs a non-aqueous electrolyte solution and utilizes a process of oxidation and reduction of lithium and lithium ions.
In the case of the above-mentioned non-aqueous electrolyte battery, however, problems exist that the non-aqueous electrolyte solution leaks out of the battery and that the non-aqueous electrolyte solution reacts with a positive electrode and a negative electrode to degrade the battery characteristics. Therefore, more recently, the spotlight is on a polymer electrolyte battery employing a polymer electrolyte.
Such a polymer electrolyte battery has conventionally generally employed a polymer electrolyte comprising a polymer such as poly(ethylene oxide) and polyvinylidene fluoride containing a solute comprising a lithium salt such as LiPF6. However, such a polymer electrolyte generally suffers low ionic conductivity, whereby sufficient battery characteristics can not be achieved.
Therefore, in recent years, a gelated polymer electrolyte comprising the above-mentioned polymer impregnated with a non-aqueous electrolyte solution obtained by dissolving a solute in an organic solvent such as carbonic ester has been utilized. In this connection, Japanese Patent Laid-Open No. 14506/1990 has proposed a polymer electrolyte comprising a gelated polymer electrolyte whose ionic conductivity is improved by a dibenzylidene sorbitol derivative with at least one ester group added thereto. Further, Japanese Patent Laid-Open No. 289040/1997 has proposed to provide a separator between a gelated polymer electrolyte and a negative electrode, so that characteristics at the interface between the gelated polymer electrolyte and the negative electrode is improved, so that charge/discharge cycle performance of the battery is improved.
Unfortunately, however, even when the gelated polymer electrolyte comprising a polymer impregnated with a non-aqueous electrolyte solution is used as a polymer electrolyte as described above, the non-aqueous electrolyte solution contained in the polymer electrolyte gradually reacts with a positive electrode and a negative electrode to decrease a battery capacity. Especially when the battery is preserved under high temperature conditions, such a reaction makes rapid progress to considerably decrease the battery capacity, whereby preservation characteristics of the battery is degraded.
Further, more recently, Japanese Patent Laid-Open No. 208743/1998 discloses to use an electrolyte solution containing a solvent selected from the group consisting of a number of solvents such as propylene carbonate and ethylene carbonate along with a specific positive electrode material. As one of such selectable solvents, vinylene carbonate is exemplified.
However, the above-mentioned official gazette discloses no operative example using an electrolyte solution containing vinylene carbonate as a solvent. Also, it does not disclose nor suggest an amount of vinylene carbonate contained in the electrolyte solution. The above-mentioned official gazette merely exemplifies vinylene carbonate. There is no reference to the influence on a polymer electrolyte battery from vinylene carbonate contained in the electrolyte solution.
An object of the present invention is to restrain, in a polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte containing a non-aqueous electrolyte solution, the non-aqueous electrolyte solution from gradually reacting with the positive electrode and the negative electrode to decrease the battery capacity.
Another object of the invention is to restrain, in the polymer electrolyte battery as described above, the decrease in a battery capacity when the battery in either a charged state or a discharged state is preserved under high temperature conditions.
A polymer electrolyte battery according to the present invention is a polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte containing a non-aqueous electrolyte solution, wherein a solvent in the above-mentioned non-aqueous electrolyte solution contains vinylene carbonate shown in the following structural formula (1) in a concentration of 0.01 to 90 vol %. 
As in the polymer electrolyte battery according to the present invention, when the solvent in the non-aqueous electrolyte solution contains vinylene carbonate in a concentration of 0.01 to 90 vol %, vinylene carbonate reacts with the positive electrode and the negative electrode at surfaces of the positive electrode and the negative electrode, to form stable layers with excellent ionic conductivity on interfaces between the polymer electrolyte and the positive electrode and between the polymer electrolyte and the negative electrode. The layers restrain the non-aqueous electrolyte solution from gradually reacting with the positive electrode and the negative electrode and hence, even when the battery is preserved under high temperature conditions, the decrease in the battery capacity is restrained, whereby preservation characteristics of the battery is improved.
The reason why the content of vinylene carbonate is set to 0.01 to 90 vol % is that when vinylene carbonate is contained in the concentration of less than 0.01 vol %, stable layers with excellent ionic conductivity can not be successfully formed at interfaces between the polymer electrolyte and the positive electrode and between the polymer electrolyte and the negative electrode and hence, preservation characteristics of the polymer electrolyte battery can not be sufficiently improved. On the other hand, when vinylene carbonate is contained in the concentration of more than 90 vol %, a viscosity of the non-aqueous electrolyte solution is made high to degrade the ionic conductivity of the non-aqueous electrolyte solution, whereby battery characteristics of the polymer electrolyte battery are degraded. Therefore, it is preferable that the solvent in the non-aqueous solution contains vinylene carbonate in a concentration of 0.1 to 80 vol %.
Further, the polymer electrolyte battery according to the present invention may employ as a solvent contained in the non-aqueous electrolyte solution in the polymer electrolyte any solvent containing vinylene carbonate in a concentration of 0.01 to 90 vol % as described above. Vinylene carbonate may be used in combination with other known solvents that have been generally utilized. Examples of such solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane, and the like.
Furthermore, in the polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte containing a non-aqueous electrolyte solution, when the solvent containing ethylene carbonate, diethyl carbonate, and vinylene carbonate is used as a solvent in the non-aqueous electrolyte solution, decrease in the battery capacity is prevented and sufficient discharge capacity is obtained even in a case where the battery in a discharged state is preserved under high temperature conditions, after which the battery is subjected to another charging to be used. The reason for this is conceivably that when the solvent in the non-aqueous electrolyte solution contains ethylene carbonate, diethyl carbonate, and vinylene carbonate, stable layers are formed on interfaces between the polymer electrolyte and the positive electrode and between the polymer electrolyte and the negative electrode so that the positive electrode and negative electrode are restrained from reacting with the non-aqueous electrolyte solution even in a case where the battery in a discharged state is preserved under high temperature conditions.
When the non-aqueous electrolyte solution employs the solvent containing ethylene carbonate, diethyl carbonate, and vinylene carbonate as described above, if a ratio of vinylene carbonate is too low, layers suitable for bringing this effect are not formed. On the other hand, if a ratio of vinylene carbonate is too high, the above-mentioned layers become thick, resulting in increased resistance. Therefore, the above-mentioned solvent containing ethylene carbonate, diethyl carbonate, and vinylene carbonate preferably contains vinylene carbonate in a concentration of 0.1 to 80 vol % and more preferably 0.1 to 3 vol %.
In the polymer electrolyte battery according to the present invention, a known solute that has been conventionally generally used can be used as a solute dissolved in the above-mentioned solvent contained in the non-aqueous electrolyte solution. Examples of a usable solute include lithium compounds such as LiPF6, LiBF4, LiN(C2F5SO2)2, LiAsF6, LiSbF6, LiAlF4, LiGaF4, LiInF4, LiClO4, LiN(CF3SO2)2, LiCF3SO3, LiSiF6, and LiC(C2F5SO2)3; and a mixture of these. Particularly, in order to further improve the preservation characteristics of the battery, it is preferable to use a compound containing fluorine as a solute. When the compound containing fluorine is used as a solute, it is conceived that more stable layers are formed on interfaces between the polymer electrolyte and the positive electrode and between the polymer electrolyte and the negative electrode to further restrain the positive electrode and negative electrode from reacting with the non-aqueous electrolyte solution.
Further, in the polymer electrolyte battery according to the present invention, it is possible to use, as a polymer for use in a polymer electrolyte containing the above-mentioned non-aqueous electrolyte solution, a known polymer that has been conventionally generally used. Examples of a usable polymer include a polystyrene-polyethylene oxide copolymer, polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, and the like. Particularly, in order to further improve the preservation characteristics of the battery, it is preferable to use a polymer having a polyethlene oxide chain. When the polymer having a polyethlene oxide chain is used, it is considered that the polyethlene oxide chain is partially broken to react with vinylene carbonate thereby forming a more stable layer on a surface of the negative electrode in a charged state and hence, the positive electrode and the negative electrode is further restrained from reacting with the non-aqueous electrolyte solution.
Furthermore, in the polymer electrolyte battery according to the present invention, materials for composing its positive electrode and negative electrode are not particularly limited, and known materials generally utilized in polymer electrolyte batteries may be used.
Examples of the above-mentioned positive electrode material for composing the positive electrode include a manganese dioxide, a lithium-containing manganese oxide, a lithium-containing cobalt oxide, a lithium-containing vanadium oxide, a lithium-containing nickel oxide, a lithium-containing iron oxide, a lithium-containing chromium oxide, a lithium-containing titanium oxide, and the like.
On the other hand, examples of the above-mentioned negative electrode material for composing the negative electrode include lithium metals; lithium alloys such as Lixe2x80x94Al, Lixe2x80x94In, Lixe2x80x94Sn, Lixe2x80x94Pb, Lixe2x80x94Bi, Lixe2x80x94Ga, Lixe2x80x94Sr, Lixe2x80x94Si, Lixe2x80x94Zn, Lixe2x80x94Cd, Lixe2x80x94Ca, and Lixe2x80x94Ba; carbon materials capable of occluding and discharging lithium ions such as graphite, coke, and calcined organic substances; and metal oxides having lower potentials than the positive electrode material, such as Li4Ti5O12, TiO2, Nb2O5, Fe2O3, MoO2, MoO3, WO2, WO3, SnO2, SnO, SiO2, and SiO. Particularly, in order to further improve the preservation characteristics of the battery, it is preferable to use the above-mentioned carbon materials or metal oxides. The reason for this is conceivably that large surface areas of carbon materials and metal oxides contribute to the remarkable effect of the above-mentioned layers, and the carbon materials or metal oxides react with vinylene carbonate in the non-aqueous electrolyte solution to form more stable layers.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.